A complete revision summary of both the topics of tectonics and coastal landscapes. It covers the key processes and meets the demand of the specification. The revision notes are condensed making it perfect to glance at before an exam.
Tectonic processes and hazards; coastal landscapes and change
May 12, 2020
11
2018/2019
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geography
tectonics
coasts
dynamic landscapes
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Tectonics
When do natural hazards become disasters?
- risk = hazard x vulnerability / capacity to cope
- PAR model = root causes – dynamic pressures – unsafe conditions = disaster = natural hazard
- a natural disaster when more than 10 dead
Distribution of tectonic hazards
- convergent, divergent and transform
- 70% of all earthquakes in Ring of Fire
- OFZ (Oceanic Fracture Zone) – belt of activity through constructive boundaries in oceans along mid-ocean ridges
- CFZ (Continental Fracture Zone) – belt of activity following mountain ranges from Spain, Alps, Middle East, Himalayas
- 500 active volcanoes and 50 erupt each year
- 5% of earthquakes are intra-plate earthquakes - due to old faults and large faults
Hotspot volcanoes
- heat rises as a hot thermal plume from deep Earth
- high heat and lower pressure at the base of lithosphere enables melting of rock
- rises and forms active volcanoes
- as the plate moves away from stationary hotspot the volcanoes are rafted away and new ones form in their place
- the old volcanoes cool and subside producing older islands, atolls and seamounts
Theory of tectonics
- inner core = 6000 Degrees Celsius, solid and mainly consists of iron
- outer core = semi molten, 4500 – 6000 Degrees Celsius
- mantle = widest layer, 870 Degrees Celsius, phases of liquid and solid in layers
- crust – oceanic = basalt, density: 3.3 g/cm3 and continental = granite, density: 2.7 g/cm3
- evidence = earthquake depths, geological fit, continental fit, fossil records, glaciation evidence, palaeomagnetism etc
Why do plates move?
- convection currents = heat from radioactive decay drives convection currents which moves plates
- slab pull = convection and gravity (denser plate pulled down) driving tectonic plate movement
- subduction = as a new crust is being created, it’s being destroyed in another by subduction
- seafloor spreading = magma forced up from the asthenosphere and hardens forming new crust, pushes plates apart
Plate boundaries
Divergent
- magma rises due to the decomposition in asthenosphere, causes gap to be filled by lava from fissures
- rift valley: when plates move apart crust breaks into parallel cracks, land between faults collapse forming steep valleys
- small and effusive eruptions – erupted basalt lava has a low gas content and high viscosity
- shallow earthquakes, less than 60km deep and low magnitude
Convergent – 2 continental
- plates move together and are pushed up
- subduction caused when the compressed sediments result in plate subduction beneath them
- shallow, magnitude, infrequent earthquakes – cut by huge thrusts
Convergent – 2 oceanic
- denser oceanic plate subducted creating friction causing earthquakes in the Benioff Zone
- magma rises to surface and breaks friction – causing volcanoes
, - frequent earthquakes and violent eruptions from curving chain of volcanic islands
Convergent – 1 oceanic 1 continental
- oceanic plate subducted and melts – deep ocean trenches mark the place where the oceanic plume starts to sink
- plumes of melted magma rise under pressure
- volcanic eruptions created by melting oceanic plate pushes up through the surface faults
- frequent large earthquakes and violent eruptions as generates magma with high gas and silica content
Transform
- 2 plates either pass each other in opposite directions or at different speeds
- friction builds up and stress released – sends shock waves through Earth’s crust
- shallow focus earthquakes = high magnitude
Volcanic hazards
- lava flows = streams of lava that have erupted from a volcano
- pyroclastic flows = mixture of dense rocks, lava, ash and gases ejected from volcano – travel at 100km per hour
- tephra + ash falls = ejected rock fragments - can cause poor visibility, slippery roads, stop engines
- gas eruptions = magma contains dissolved gases that are released i.e. water vapour, sulphur dioxide
- lahars = volcanic mudflows composed of fine sand and silt – if the eruption melts ice or heavy rainfall which erodes
rock and soil
- Jokulhlaup = heat of eruption can melt the snow and ice in a glacier causing heavy and sudden floods – the water
carries glacial moraines and blocks of ice
Volcanic Explosivity Index – VEI
- used to describe and compare size and magnitude of eruptions
- 0 to 8 on logarithmic scale
- 0 to 3 = constructive plate margins and mid-plate hotspots
- 4 to 7 = destructive plate boundary
- 8 = supervolcano
Why are some volcanoes more hazardous than others?
- greater viscosity = more explosive eruption – viscosity determined by temperature, dissolved gases and chemistry
- proximity to population centres
- predictions, forecasts and reactions used – i.e. geologists use evidence from past and create hazard maps
- plate margins – more hazardous at destructive i.e. 80% of world’s eruptions occur at destructive
Measures of earthquakes
Richter Scale
- based on amplitude of the lines made on a seismograph using largest wave amplitude recorded
- pros = accurate, objective, can compare
- cons = scale goes up to 8, logarithmic – may lose accuracy, doesn’t take into consideration impacts
Mercalli
- intensity
- scale I to XII
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